Oh great, a fundamentalist Muslim nation has enough fissile material for a nuclear weapon... can you picture the wonderful potential outcomes from this one?
1) The U.S. decides to invade because, well, that's what we do to people we think have these things.
2) Israel does something astoundingly stupid, like bombing the Iranian nuclear site and starting yet another shooting war in the Middle East.
3) Nothing, because we weren't right in the first place.
4) Iran builds their bomb, and uses it somewhere, sometime... or just hands it over to Al-Qaeda.
5) Insert scenario here...